A major cause of stress-induced birefringence is the stresses in the optical waveguides resulting from the mismatch in the thermal expansion coefficients between the substrate and the waveguide material(s). This causes the refractive index and the light propagation properties to be dependent upon the direction of polarization and hence polarization dependent loss.
Conventional methods to reduce the stress in waveguides involve using substrate material with thermal expansion coefficients closely matching that of the core.
U.S. Publication No. 2002/0074308 A1 to Beguin describes a method of manufacturing a planar waveguide with a core and overclad layers.
U.S. Publication No. 2002/0097962 A1 to Yoshimura et al. describes single and multilayer waveguides and processes to fabricate them.
U.S. Pat. No. 6,421,472 B1 to Morani et al. describes an athermalized polymer overclad integrated planar optical waveguide device and method.
U.S. Pat. No. 5,612,171 to Bhagavatula describes planar optical waveguides with planar optical elements.